The invention concerns integral porous separation membranes supported on nonwoven polymeric supports such as are typically employed as filter media in conventional filtration modules for the filtration of fluids.
The term "porous separation membranes" is generally understood to refer to ultrafiltration and microfiltration membranes that have pores with diameters of about 0.001 to 0.1 .mu.m and about 0.01 to 30 .mu.m, respectively. Such membranes are conventionally fabricated from organic polymers, and are often brittle, with little capacity for mechanical loads. For increasing mechanical structural strength, porous membranes of organic polymers are supported with a wide variety of support materials such as woven material, knitted fabrics, non-woven materials, or films. The terms "integral" and "nonintegral" are differentiated by the type of bonding between the membrane and the support material.
The term "integral supported membranes," refers to those wherein the still fluid permselective membrane material is brought into contact with the support material and the membrane is formed by the mutually interpenetrating bond formed between the permselective membrane and the support. The support then, to a certain extent, penetrates into the membrane or may even be entirely encapsulated by the membrane. The membrane can be reinforced on one or both sides.
In contradistinction, in the case of a non-supported membrane, the already cast permselective membrane is applied directly onto the support by, for instance, lamination or by an adhesive. From the viewpoints of production and applications, integral supported separation membranes are preferable.
The cost of the production of integral supported separation membranes is governed by the degree of increase of mechanical strength desired, but increased mechanical strength is typically attainable at the cost of loss of service life or filtration capacity, often up to some 40% and usually accompanied by a decrease in retention capability for the substances to be removed, all as compared to non-supported membranes. Such drawbacks are unacceptable in industrial applications.
Thus, the object of the invention is the provisions of integral supported membranes having longer service lifetimes, higher filtration capacity and improved retention capability. This object and others which will become apparent to one of ordinary skill are met by the present invention, which is summarized and described in detail below.